Mechanical oscillator of constant-frequency value



Jqne 17, 1930. A. KAROLUS 1,763,853

MECHANICAL OSCILLATOR OF CONSTANT FREQUEfiCY VALUE Filed Nov. 25, 1927 aoefficient pf thermal expanswn a elael'zcilg Patented June 17, 1930 UNITED STATES PATENT- OFFICE 4 AUGUST KAROLUS, OF LEIPZIG, GERMANY, ASSIGNOR TO RADIO CORPORATION OF AMERICA, A CORPORATION OF DELAWARE MECHANICAL OSCILLATOR OF CONSTANT-FREQUENCY VALUE Application filed November 25, 1927, Serial No. 235,761, and in Germany December 22, 1926.

The invention relates toa method to keep the frequency of mechanical oscillators, which are required to be of great accuracy, constant. The same is applicable, for in 5 stance, to tuning forks which serve for the purpose of accurate synchronization of drives of allkin'ds, for instance, for picture transmission; furthermore, for pilot tubes which are expected to keep the frequency of an oscillation system, such as a valve transmitter, at a constant value. It is known to the prior art that tuning fork oscillators as time tappers may be used for purposes of the said sort in various circuit arrangements. In

order that in such a. scheme the highest possible constancy of frequency may be insured,

the aim generally has been to make tuning forks of this kind from a sort of material exhibiting the lowest possible coefficient of expansion. The idea was that the thermal coefficient of expansion would diminish the frequency with rising temperature as a re sult of longitudinal changes and the shifting of particles of the vibrating parts associated therewith, and that the said coefficient must for this reason be kept as low as possible.

The temperature coefficient of frequency in the case of a tuning fork made from ordinary steel ranges approximately between 1 and 2 x 10- Sincein perfect synchronization in picture transmission comprising the use of local time tapper oscillators independent of each other makes at the most a departure of 1/100,000 permissible, it follows that on the basis of the above temperature coefficient of frequency it would be necessary to keep the temperature of tuning forks constant to with in 1/10 of one degree. This is very difficult in practice, and possibleonly with very complicated means.

It .is an object of the disclosure of the present invention to insure constancy of frequency to a far greater degree even in the presence of greater temperature changes and by the aid of simpler means. To the end of precluding the temperature influence upon the vibrating tuning fork, according to former practice, materials of-very low coefficientsof expansion were used, such as the well known materials known as invar or indilatans. lVhereas the selection of such a material in the case of pendulunis, which are dependent for their period of vibration only upon the length, has been attended with success, it has proved a failure for such mechanical oscillators whose frequency is governed both by mass and elasticity. For instance, in the case of tuning forks made from such material. it has been found that the temperature coefficient of frequency turns out to be even greater than for ordinary grades of steel; indeed, it may range up to 3 x 10".

The underlying reason is that while the longitudinal extension becomes vanishingly small with the temperature, the existent temperature coefficient of elasticity substantially remains unvaried. This so-called thermoelastic coefiicient is negative for such substances as for instance, invar. Generally speaking, in the case of positive coefficients of expansion, a negative thermo-elastic coefficient means that the action of a decrease in elasticity of the material and that of a growth in length and attendant shifting of masses upon the frequency become added; indeed,

the ensuing frequency decrease is the joint result of both influences. An oscillator niaterial with vanishingly low coefficient of expansion, for instance, invar, as before pointed out, may exercise'by virtue of its thermoelastic coefficient a more marked frequency influence with temperaturethan in the case of a material'having an appreciable thermal extension in length as ordinary steel.

The present invention is based upon the clear appreciation of the said conditions, and the same essentially resides in that for the making-of tuning forks or other mechanical oscillators materials are used whose thermal coefficient of expansion and thermoelastic coefficient have the same sign. For instance, if the same is positive, then any de- One way to that end consists in that the tuning forks or other oscillators are made. from uniform material in which the coeflicient of expansion and the thermo-elastic coefficient are both positive at the proper relationship. As an example of a grade of steel suitable in the above'sense may be mentioned a chromenickel steel alloy, containing about 30% nickel and from 10 to 12% chromium, it being presupposed that the distribution of masses or particles is similar to that of an ordinary prong-type tuning fork, while for other shapes of the oscillators other alloying proportions may be found.

Of course, fundamentally speaking, the invention, consisting essentially, as shown by the single figure of the drawing in the selection of materials of coeificients of expansion and thermo-elasticity of equal sign may be practiced also in this way that substances are chosen in which both coefiicients are negative.

Another way of carrying the basic idea of the invention into practice consists in that the oscillator is not made from a uniform alloy, but is composed of different metals. By choosing a plurality of metals of different thermal longitudinal expansion or different thermo-elastic coeflicient, conditions may be made so that the compensation is practically perfect. In this connection, another improvement is insured by that not only different materials are composed in a purely additive way, but that the distribution of ma terials and masses is chosen in a special way; for the influence of each mass possessing a definite variation in thermal respect, in

length and elasticity, will differ according to its local arrangement within the oscillating structure or system.

Having now described my invention, what I claim is:

1. A mechanical vibrator for maintaining constant frequency under varying conditions of temperature which is formed from a metallic substance whose coefficients of expansion and elasticity are of like sign.

2. A mechanical vibrator for maintaining constant frequency under varying conditions of temperature which is formed from a ferrous alloy whose coefiicients of expansion and elasticity are of like sign.

3. A mechanical oscillator having a coefficient of thermal expansion of one sign and a coefficient of elasticity of like sign, whereby temperature variations tending to change the frequency of said oscillator are compensated for by changes in the elastic erties of the said oscillator.

' 4. A tuning fork formed of a ferrous alloy inv which changes in length of said fork due to variations in temperature, are compensated for by changes in the elastic properties of the said ferrous alloy.

5. A tuning fork formed of chrome-nickel steel whose coeflicients of thermal expansion and elasticity are of like sign.

6. A mechanical vibrator for maintaining constant frequency under varying conditions of temperature which is formed from a homogeneous metallic substance whose thermal coeificient of expansion and thermo-elas-- AUGUST KAROLUS. 

